1. Field of the Invention
This invention relates to reading an image while moving an image taking device such as a CCD (charge coupled device) sensor, thereby obtaining a picture of high resolution.
2. Description of the Related Art
An image reader using an image taking device such as a CCD sensor has been rapidly put into wide use, and recently there is a demand for an image reader having a higher resolving power.
Resolving power of an image reader can be improved in various ways. For example, as disclosed in Japanese Unexamined Patent Publication Nos. 64(1989)-863, 53(1978)-101939 and 1(1989)-221065, by changing relative positions of picture elements of a CCD with time, thereby increasing sampling points, a high resolution picture can be obtained with a CCD having a relatively small number of picture elements. For example, shifting the CCD by a half of the picture element pitch in both the horizontal and vertical directions is equivalent to doubling the picture elements of the CCD in both the horizontal and vertical directions.
When the CCD is shifted in both the horizontal and vertical directions as described above, the image of one screen is formed by four frames which are sampled in different positions.
The resolving power of the image reader can be improved by moving the sampling-position as described above. However since the CCD stores signal charges even during shift, the resolving power cannot simply depend upon the number of picture elements.
Such a deterioration in the resolving power due to shift of the sampling position can be avoided by not using the signal charges stored during the shift, e.g., by intermittently reading an image in synchronization with the shift of the sampling position. However this approach gives rise to a problem that it takes a longer time to read the image since the image reading is repeatedly interrupted.
The deterioration in the resolving power can also be avoided by idly reading one frame image information after the sampling position shift or by discharging or wasting signal charges of one field. However either of the methods also gives rise to a problem that it takes a longer time to read image information for each screen.